Despite the fact that chronic low back pain is a very costly prevalent problem, little is known about the exact etiology of this disorder. Research evidence, however, has implicated the role of behavioral and psychosocial factors and aberrant lumbar muscle function in its development and maintenance. The proposed investigation will test several hypotheses that stem from the operant model of chronic low back pain and examine associations between environmental stimuli and measures of impairment, pain behavior and aberrant muscle function. One-hundred patients with chronic idiopathic low back pain and their spouses/significant others (SPOSOs) will be recruited. SPOSOs will be classified as solicitous or non-solicitous according to their mates responses on the West Haven-Yale Multidimensional Pain Inventory. Patients and SPOSOs will be exposed to three experimental conditions, SPOSO absent, SPOSO present, and conflict-discussion while patients' trunk mobility and strength are evaluated by a back-testing device. Objective measures of impairment and pain behavior will include biomechanical parameters such as trunk range of motion, velocity, and torque, as well as video-tape ratings of pain behavior. Lumbar paravertebral EMG will also be assessed during static trunk flexion under each of the experimental conditions to evaluate the flexion- relaxation phenomenon in relation to impairment, pain behavior and environmental factors. In addition, the relation between laboratory assessments and patients' functional activities in the natural environment will be determined. The anticipated results are likely to address theoretical questions about the significance of the operant model in the maintenance of impairment and pain behavior, and link these variables to aberrant muscle function. Also, the results are likely to have important implications for clinical assessment and treatment of patients insofar as they demonstrate the controlling influence of behavioral and psycho- social factors in the display of impairment and pain behavior.